1. Field of the Invention
This invention relates to a method of single-stage galvanizing of an iron or steel material with a molten zinc-aluminum alloy using a molten flux.
2. Background Information
In recent years, there is seen a growing demand for galvanizing with a molten zinc alloy of high aluminum content that shows sufficiently high corrosion resistance to withstand hostile corrosive environments and provides maintenance-free coatings. One main problem with galvanizing using a molten zinc-aluminum alloy is that black spottings (ungalvanized areas) and poorly adherent zinc-aluminum alloy layers are highly prone to occur. To deal with this problem, various galvanizing methods have been proposed, including a dry galvanizing method using a gas reduction technique, a two-stage galvanizing method and a wet galvanizing method using a zinc chloride free flux.
However, most of the galvanizing methods proposed so far are incapable of solving the aforementioned problem with galvanizing with a molten zinc alloy of high aluminum content, particularly from the viewpoint of practical applicability. First of all, dry galvanizing in a reducing gas atmosphere without using a flux has no flexibility in the materials that can be treated, since it is applicable only to the galvanizing of steel strips and wires by a continuous dipping method. In addition, large-scale production facilities are required.
Galvanizing with a molten zinc alloy can be performed in an air atmosphere using a flux. To achieve high quality galvanizing with a zinc bath of high aluminum content by an improved version of this method Examined Japanese Patent Publication (kokoku) No. 19299/1992 proposed a two-stage process which consists of ordinary galvanizing with molten zinc, followed by galvanizing with a molten zinc-aluminum alloy. However, this process is not highly cost effective from the viewpoints of facilities and operating efficiency.
The ordinary methods of galvanizing with molten zinc using a flux can be classified into a wet system and a dry system. In the wet system, a molten zinc bath is covered with a blanket molten flux layer chiefly made of zinc chloride and an iron or steel material that has been subjected to preliminary treatments to remove the oxide film is passed through the blanket flux layer to be dipped into the molten zinc bath so that it is galvanized to obtain a zinc coating. The currently used molten zinc alloy bath of high aluminum content contains either 5% or 55% of aluminum. At the interface between the flux and the molten alloy bath zinc chloride present in the blanket flux reacts with the aluminum in the bath, according to the reaction formula: 3ZnCl.sub.2 +2Al=2AlCl.sub.3 +3Zn, thereby forming volatile aluminum chloride; the resulting aluminum chloride evaporates into the atmosphere to a cause partial loss of the aluminum ingredient of the bath while, at the same time, it prevents the galvanized layer from adhering firmly to the entire surface of the steel material (i.e., some areas remain ungalvanized with the alloy coating). To solve this problem, Japanese Patent No. 2510361 has proposed that a hot galvanizing bath consisting of 40-80% aluminum and zinc be used with a flux composition based on an alkali metal-aluminum fluoride (for example cryolite) rather than a zinc chloride-containing flux.
The wet system using a blanket molten flux that floats on the alloy bath has another problem. After the galvanizing, the galvanized article is passed through the blanket flux layer in order to be withdrawn from the galvanizing bath. As a result, the flux is prone to adhere to the surface of the galvanized layer and particularly in the case where it is chiefly made of an alkali metal-aluminum fluoride, the flux deposit is water-insoluble and therefore is not easy to remove unless certain post-treatments are applied, but then the resulting surface does not have a silvery white gloss.
In the dry galvanizing system, a steel material to be galvanized is immersed in an aqueous flux solution in a separate vessel to form a flux coating on the surface of the material to be galvanized, which is then dried and immersed in a molten zinc alloy bath. This method also has several problems. If an article to be galvanized is not thoroughly dried, black spottings (ungalvanized areas) or poorly adhered layers are prone to occur. In addition, as in the aforementioned wet system, if zinc chloride is present in the flux, the unwanted aluminum chloride will form. To solve this problem, it has been proposed to replace a portion or all of the zinc chloride with tin chloride which has a substantial surface activating action but which is expensive [see, for example, Unexamined Published Japanese Patent Application (kokai) No. 146651/191]. It has also been proposed that an organic salt be added to the flux with a view to providing better wettability [see, for example, Unexamined Published Japanese Patent Application (kokai) No. 233459/1995]. However, this approach does not provide a satisfactorily firmly adhered layer in galvanizing with the aforementioned molten zinc alloy of high aluminum content and, as a matter of fact, in an experiment made by the present inventors, black spottings (ungalvanized areas) often occurred and problems were also encountered with the operating efficiency and cost.